174 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



This species was discovered by Mr. Ernest Ingersoll, Naturalist of the 

 United States Geological Survey of the Territories, under Professor Hayden. 

 It can scarcely be compared with any known North American species. 



At first sight I was disposed to consider the species a Zonites, but examina- 

 tion of the animal proved it to belong to the Helicea. 



Jaw low, wide, slightly arcuate, ends slightly attenuated ; whole anterior sur- 

 face with about 22 broad, flat, slightly separated ribs, whose ends denticulate 

 either mai'gin. 



Lingual membrane long and narrow. Teeth about 16 1 16. Centrals as 

 usual in the Helicea (PI. III. Fig. V). The side cusps and cutting points are 

 well developed, the base of attachment longer than wide. Laterals of same 

 type, but asymmetrical, and consequently only bicuspid. The change from 

 laterals to marginals (8th and 9th teeth of figure) is very gradual, there being 

 no splitting of the inner cutting point. Marginals (16th tooth of figure) very 

 low, wide, with one inner, long, blunt cutting point, and one outer, small, 

 blunt. The low, wide marginal teeth of this species are peculiar. 



SPURIOUS SPECIES OF MICROPHYSA. 



Microphysa minuscula of Von Martens (Alb., ed. 2) is a Zonites (q. T.). 



HEMITROCHUS, SwAlNSON. 



Animal heliciform (of //. various), stout, anteriorly blunt, posteriorly long, 

 acutely terminating; mantle central, thin, simple, protected by a shell; no dis- 

 tinct locomotive disk ; no caudal mucus pore ; 

 Fig. 83. 



respiratory and anal orifices subcentral, on the 



right side of the mantle, under the peristome 

 of the shell ; generative orifice not observed, 

 probably behind the right eye-peduncle. 



Shell external, with the perforation open 

 Animal of H. variant. or c l ose( j, globose, shining ; spire short ; whorls 



4-5, the last large, deflexed at the aperture ; columella dilated at the base ; 

 aperture contracted, subvertical, roundly lunate ; peristome simple, obtuse, la- 

 biate within, its margins distant. 



A West-Indian genus; one species has been introduced into the Florida 

 Subregion. 



In Ann. Lye. N. II. of N. Y., X. 341, I have, in connection with my friend 

 Mr. Bland, shown the necessity of using this name in preference to Polymita. 

 I will here simply repeat that the type of the latter genus is muscarum, Lea, 

 from which the other species formerly associated with it differ generically 

 in dentition. They will therefore be known by the first published name, 

 Hemitrochus. 



